Template:Infobox Lost character Desmond David Hume is a fictional character on the ABC television series Lost portrayed by Henry Ian Cusick. Desmond was not a passenger of Flight 815. He had been stranded on the island three years prior to the crash as the result of a shipwreck.

Desmond attended a university, but failed to graduate due to his support being needed by his three brothers when their father left them. He also worked as a set designer for the Royal Shakespeare Company. Desmond was engaged to a woman named Ruth for six years. One week before the wedding, Desmond got cold feet and passed out due to binge-drinking. He woke up to find a monk named Brother Campbell, and Desmond decided to join the monastery, feeling a higher calling. While labeling wine bottles one day, Desmond is assaulted by Ruth's brother, prompting him to pay her a visit. He explains to her about his higher calling, but she angrily calls him a coward for jilting her. That night, Desmond breaks into the wine cellar and starts drinking when he is dismissed by Brother Campbell, who has some connection to the mysterious older woman Desmond encounters during his trip back in time. He offers him to catch a ride into town with Penny, who delivers the wine for them in Carlisle. Two years after meeting, the two decide to move into an apartment together.

Desmond visits Penny's father Charles to ask for his daughter's hand in marriage. While appreciative of the bold gesture, Charles makes clear his disapproval of Desmond, saying he is not good enough for her. After meeting and conversing with Mrs. Hawking; who tries to correct his timeline; Desmond realizes this when he is unable to pay for a photograph, and breaks up with Penny. In an attempt to prove himself to Charles, Desmond joins the Royal Scots Regiment of the British Army.

While in training, he reverts to the future at the island and is completely confused. After a brief period with Sayid, he goes back to the past. He travels through time a few more times, understanding more and more about his situation with the help of Faraday during a two-day leave he has from his military service. Faraday explains to Desmond that he must have an anchor, or constant, something that is familiar in both 1996 and 2004, and he must make contact with this constant in the future to stop the jumping back and forth between time. If he doesn't, Faraday tells Desmond that he will probably experience a brain aneurism and die. Desmond then makes contact with Penny and asks for her phone number so he can call her in the future. She is reluctant to give him the number, but he promises he won't call for eight years - until December 24, 2004. She relents, gives him the number, and then Desmond leaves.

At some point between 1996 and 2000, Desmond is put in prison. The reason for this is still unknown.

Following his release from prison, Desmond is dishonorably discharged and given his belongings, one of which is the book Our Mutual Friend. Outside the prison, Desmond is greeted by Charles, who admits to confiscating all the letters Desmond wrote to Penny, and pays him off to leave her alone. Desmond travels to America, where he meets Libby. We find out in their conversation that Desmond did not accept the money from Widmore. She gives him her boat when he reveals his plan to win a boating race, funded by Charles Widmore.

Before setting off, Desmond goes to train at a stadium in Los Angeles; where he is confronted by Penny. She is upset that he has not contacted her after getting out of prison. He asks her to wait for him for one more year, when the race will have ended. He encounters Jack at the stadium and offers him some encouragement before resuming his training. After he enters the race, his boat (the Elizabeth) is caught in a ferocious storm, and he is eventually knocked unconscious.

Sometime in 2001 (The show is set in 2004, and Desmond has said that he's been stranded on the island for three years), Desmond washes ashore on the island without his boat. A man named Kelvin Joe Inman emerges from the jungle in a HAZMAT suit and takes him back to the hatch. He asks if Desmond is "him" and is upset when he realizes that he is not. Desmond watches as Kelvin inputs the numbers into a computer. When Desmond asked, "What was all that about?" Kelvin responded, "Just saving the world." He tells Desmond to inoculate himself with an unnamed vaccine every nine days, since he was out in the "quarantined" island, and may be infected.

Kelvin trains Desmond about the operations of the hatch, including the button (which he explains safely discharges an unusually strong magnetic fluctuation situated beneath them) and ways to trigger a lockdown. Desmond also witnesses Kelvin painting the blacklight picture on the bulkhead doors, which was started by Kelvin's old partner, Radzinsky. Desmond asks what happened to him. Kelvin points to a stain on the ceiling, explaining it is all that is left of Radzinsky, as this is the spot where he took his own life.

Three years pass, and Desmond desperately wants to go above ground, but Kelvin never allows it, although he himself leaves for hours each day in his HAZMAT suit.

Desmond catches Kelvin drunk one night in a secret crawlspace below the floor, dangling a key above a fail-safe mechanism. Kelvin explains that if the fail-safe mechanism is activated, the hatch will be destroyed, destroying the electromagnetic anomaly with it.

When Kelvin leaves one day, Desmond notices that Kelvin's HAZMAT suit has a tear on its leg. He follows Kelvin above ground, where he discovers Kelvin removing the suit and finds the air is safe to breathe. Desmond follows him to a cove, where he sees his sailboat in perfect shape. Kelvin had been leaving the hatch to fix the boat a little each day, planning to escape the island and leave Desmond behind. Kelvin startles Desmond and tells him that he knew he was being followed, and then invites Desmond to escape with him, but Desmond is worried about the button. After Kelvin expresses his doubts about the validity of the button, Desmond becomes enraged that he may have spent three years of his life on the island unnecessarily and attacks Kelvin. They struggle, and Desmond accidentally smashes Kelvin's head on a rock, killing him. Desmond takes the key for the fail-safe mechanism from around Kelvin's neck and races back to the hatch, where the timer has already reached zero and the computer is registering a system failure. A massive electromagnetic field builds up, attracting all metal objects to the sealed door inside the hatch, including Oceanic Flight 815. Desmond manages to stop it by inputting the code, which safely disperses the electromagnetic field.

For forty-one days, Desmond lingers in the hatch. He gradually falls into a deep depression to the point of even contemplating his own suicide. As he opens up Our Mutual Friend, it being the last book he plans to read, he finds a note that Penny had hidden inside, telling him not to despair, as well as reminding him that she will always wait for him and that she loves him. Even more depressed now, he goes into a rage and makes a mess out of the hatch. As he collapses, Desmond then hears someone shouting from the top of the hatch. Unknown to Desmond, it is Locke asking the hatch for help after Boone's fatal plane injury. When Desmond turns on a light to see who it is, Locke, thinking his prayers have been answered, quiets down. Desmond, similarly, considers the voice to be a sign that he is no longer alone, and regains hope.

When Locke, Kate and Jack enter the hatch, they accidentally damage the computer after a brief firefight with a panicked Desmond. Convinced that the world is going to end, he tries to fix the computer, but was unable to. Desmond frantically flees the hatch. Jack catches up with him; Desmond tells him the code, and to enter it every 108 minutes. He then recognizes Jack from their encounter at the stadium, and he asks him about the patient he had mentioned operating on. Desmond leaves saying, "See you in another life, yeah?" as he had in their first encounter. Although it was not shown exactly how, Desmond manages his way back to the Elizabeth and attempts to sail to Fiji. However, his plans go awry.

A drunken Desmond returns in his boat, having been unable to navigate away from the island, making him compare it and its waters to "a bloody snow globe." Later he is confronted by Locke who tells him of the station 5 (The Pearl) orientation film he and Eko saw, and Desmond begins to lose his faith in the button. While Desmond wavers back and forth with his conviction on the button, he and Locke trigger another lockdown, trapping Mr. Eko outside the computer room, and they wait for the countdown to hit zero, to see what will happen. Eko uses dynamite from the Black Rock to attempt to get back into the computer room, but he fails.

As the countdown reaches zero, Locke shows the printout he obtained from The Pearl. Desmond realizes that the date of the prior "system failure" was the same day as the plane crash, September 22, 2004. Desmond believes that his failure to push the button that day resulted in an electromagnetic field that pulled down Oceanic Flight 815. He is insistent that the button must be pressed, but Locke angrily reacts by destroying the computer. Desmond retrieves the key to the fail-safe mechanism, wanting to save Locke because Locke saved him the night he shouted at the hatch door. As Desmond uses the key, a bright white light envelops his face.

Upon turning the key, Desmond is sent back to 1996, where he relives the moments leading to his leaving Penny. He remembers the island only after seeing Charlie busking in the streets. He is told by a woman, Mrs. Hawking, that it is his destiny to be on the island. When Desmond snaps back to the present, he finds himself amidst the hatch's scattered remains, completely naked. Hurley clothes him, and Desmond tells him of Locke's speech, which is yet to occur. Upon returning to the beach, he then asks Claire to leave her shelter for the day. When she refuses, Desmond constructs a lightning rod, diverting the ensuing lightning to strike it instead of her shelter. Desmond accompanies Locke, Sayid, Nikki and Paulo to the Pearl station, where they witness a live surveillance from inside the Flame station. He leaves, moments before Eko's death. After his burial, Desmond informs Charlie and Hurley in the jungle, but leaves suddenly to rescue a drowning Claire in the ocean. In an attempt to question his clairvoyant gift, Charlie and Hurley get him drunk, but Desmond retaliates before being interrogated. He later informs Charlie of his impending death.

Desmond constantly reminds the two that he cannot simply experience visions by will, particularly when Charlie wishes to know about his demise, and also when Hurley asks him to help in the investigation of the apparent deaths of Nikki and Paulo. He does, however, inform Hurley of an argument between Nikki and Sawyer earlier that morning. The next morning, Desmond experiences more visions, and asks Charlie, Hurley and Jin to accompany him on a hike to the cable in the sand. That night, the four witness a passing helicopter, and its pilot bailing at the last minute. They trek inland, discovering various belongings of the pilot, including the photograph that Desmond and Penny took together, prompting him to believe that the pilot is actually his former fiancée. Desmond rescues Charlie from a foreseen death, which he failed to mention earlier. The four eventually find the parachutist suspended from a tree. Desmond cuts her down and removes her helmet. However, he is disappointed to find that it isn't Penny after all. She coughs out his name before passing out. When Mikhail arrives, Desmond bargains with him to help her in exchange for his freedom, having captured him. He agrees, and manages to tend to her. The four return her to the beach soon after.

Desmond suggests informing someone about Naomi's arrival, and approves of Charlie's decision to tell Sayid over Jack. He tells Sayid about Naomi's mission to find him for Penny. The next day, as Jack and Juliet lead the camp into the jungle, Desmond experiences more visions of Charlie's death, but will ensure the rescue of Claire. He tells Charlie, prompting him to sign up for entering the Looking Glass station. Desmond accompanies him, and follows the cable into the ocean. Desmond offers to take his place, thinking that he is supposed to be dying in Charlie's place, but is knocked out by Charlie. Desmond comes to, only for Mikhail to shoot at him. Desmond swims down to the station and hides in a locker. He shoots Mikhail after he kills the two resident Others, but is confused when he is unable to find his body minutes later. Charlie makes contact with Penny, and informs Desmond, but before he can enter the room, Charlie realising they would both die as he is already fated to, locks him out. Desmond watches helplessly as Mikhail blows open the window, causing Charlie to eventually drown. Moments before drowning, Charlie writes on his hand with a marker informing Desmond that the boat is not Penny's.

Desmond makes his way back to the beach, where he warns the survivors there that the people on Naomi's boat are not who they say they are. When Hurley asks about Charlie, Desmond remorsefully breaks the news to him about Charlie's death. Despite his clear mistrust in the people on the boat, Desmond chooses to remain with Jack instead of going to the Barracks with Locke; his reasons are to find answers. After questioning Frank about Naomi's picture of Penny, Desmond departs from the island to go to the freighter with Sayid and a dead Naomi.

As the helicopter that Desmond and Sayid are being transported on flies through a storm, Desmond begins having "perceptions" of another reality of himself. This reality shows Desmond ranked as a private in the British Army's Royal Scots Regiment. He begins flashing back and forth between these realities, and the shifting causes Desmond to lose all memories of the Island and Sayid, who is accompanying him on the helicopter. It is further revealed that the shifting perceptions that Desmond is experiencing are actually a form of time travel. Desmond speaks to Daniel Faraday who tells him when he reverts to the memory of him in 1996, he must go to Oxford University and find the past Faraday. Faraday was working on a time machine.

Desmond is also told that he must find a constant, something or someone that is the same in both 1996 and 2004 that he can use as a reference point. He uses Penelope as a 'constant' from the past and future. Desmond realizes that in order to save his life he must make contact with Penny on Christmas Eve 2004, which is the date back on the freighter, in order to stop the time dilation. In 1996, he manages to find an angry Penny who does not want to see him and asks her to pick up the phone on Christmas Eve 2004. When he reverts to the present, he makes the call with the help of Sayid. She answers the phone and tearfully tells him she has been searching for him and knows about the island, and that she loves him. It is also revealed that Daniel Faraday had noted in his journal that, if anything went wrong, Desmond would be his constant.

With Desmond's situation resolved, he and Sayid meet Captain Gault only moments after witnessing a female crew-member, Regina, commit suicide by jumping off the side of the freighter with a chain wrapped around her body. The captain theorizes that his crew has been suffering from cabin fever due to the close proximity of the island and would turn the ship around if it weren't for a mysterious saboteur disabling the engines. Gault tells Desmond, much to his surprise, that Widmore owns the freighter and is trying to track down Benjamin Linus. When being escorted to their new room, Desmond and Sayid see Michael on the ship, working as a janitor and using the alias, "Kevin Johnson".

Desmond and Sayid later confront Michael in the engine room where he explains to them how he came to be on the freighter. Knowing that Michael works for Ben Linus, Sayid instinctively blows his cover by taking him to Gault with Desmond.

In keeping with the show's motif of naming important characters after important philosophers (e.g., John Locke, Jean-Jacques Rousseau and Mikhail Bakunin), Desmond David Hume is named after the famous Scottish Enlightenment philosopher, economist, and historian David Hume. Hume's philosophy partially states that moral responsibility requires determinism, and these elements play a large role in Desmond's life. David Hume also believed that humans could not persist through time. In the 4th season Desmond begins to exist in 1996 and 2004; correlating with David Hume's theory against personal identity.